Heat transfer tube, air-heated evaporator and method for producing a heat transfer tube

ABSTRACT

A heat transfer tube (2), in particular a finned tube, for an air-heated evaporator (1) for heating and/or evaporating cryogenic fluids, having a tube section (8) and a coating (15) which is provided on the outer side of the tube section (8) and has a hydrophilic portion (16) and a hydrophobic portion (17), wherein the hydrophilic portion (16) forms, in the coating (15), seed points (18), which are peripherally enclosed by the hydrophobic portion (16), for condensing air moisture thereon and wherein the seed points (18) have a size of less than 100 nm.

The invention relates to a heat transfer tube, in particular a finned tube, for an air-heated evaporator for heating and/or evaporating cryogenic fluids, to an air-heated evaporator for heating and/or evaporating cryogenic fluids with such a heat transfer tube and to a method for producing such a heat transfer tube.

With the aid of air-heated evaporators, cryogenic fluids, such as for example liquid oxygen, liquid nitrogen, liquid argon, liquid hydrogen, liquid carbon dioxide and liquefied natural gas, are heated and/or evaporated with the aid of the surrounding heat. Air moisture contained in the fresh air used for the heating may condense and freeze on heat transfer tubes of such air-heated evaporators. As a result, an insulating layer forms on the heat transfer tubes and has to be manually knocked off by means of a tool or removed with the aid of a steam jet to maintain the functionality of the evaporator.

Against this background, the object of the present invention is to provide an improved heat transfer tube for an air-heated evaporator for heating and/or evaporating cryogenic fluids.

Accordingly, a heat transfer tube is proposed, in particular a finned tube, for an air-heated evaporator for heating and/or evaporating cryogenic fluids. The heat transfer tube comprises a tube portion and a coating, which is provided on the outer side of the tube portion and has a hydrophilic component and a hydrophobic component, the hydrophilic component forming in the coating seed points, which are peripherally enclosed by the hydrophobic component, for condensing air moisture on the same and the seed points having a size of less than 100 nm.

By providing the seed points, the condensation of the air moisture contained in the fresh air on the heat transfer tube can be controlled in such a way as to promote the formation of spherical ice crystals that grow up in layers. By virtue of the wettability of the seed points and the unwettability of the hydrophobic component, the ice crystals are only in contact with the heat transfer tube at a very small portion of their surface. As a result, they can be removed particularly easily from the heat transfer tube. The formation of an insulating layer is prevented by the specifically intended crystallization, since from an early stage the ice crystals can no longer attach themselves to the heat transfer tube and fall off of their own accord or are removed from the tube by the flow of fresh air passing over the heat transfer tube. This makes it possible to dispense with manual removal of an insulating layer, such as for example a crust of ice, thereby eliminating the possibility of the heat transfer tube being mechanically damaged. It is also possible to dispense with energy-intensive removal of the ice by means of steam jets. The seed points are preferably nanoparticles. The term nanoparticles refers to clusters of a few to a few thousand atoms or molecules. The term nano relates to their size, which is typically 1 to 100 nm. The seed points are preferably 10 to 90 nm in size, more preferably 20 to 80 nm, more preferably 30 to 70 nm, more preferably 40 to 60 nm. The size of the seed points may be a diameter, a length, a height and/or a width of the same. The size may also be referred to as the particle size. The heat transfer tube may have heat transfer ribs extending out radially from the tube portion. The heat transfer tube may then also be referred to as a finned tube. Alternatively, the heat transfer tube may not have any ribs, and therefore be smooth.

According to one embodiment, the seed points are so small that ice crystals formed at the seed points are spherical.

The ice crystals form from air moisture condensed at the seed points. In this case, the ice crystals grow up in layers. A spherical geometry of the ice crystals forms in this case because of the surface tension of water. As the growth of the ice crystals progresses, the surface area with which the ice crystals attach themselves to the seed points becomes increasingly smaller in comparison with an overall surface of the ice crystals, so that they fall off under even the slightest contact with the heat transfer tube.

According to a further embodiment, the seed points are so small that the ice crystals fall off from the heat transfer tube under their own weight.

The heat transfer tube is preferably arranged in such a way that the gravitational force is oriented parallel to the coating. The ice crystals preferably keep growing until they fall off from the heat transfer tube of their own accord.

According to a further embodiment, a diameter of the seed points is smaller than a diameter of the ice crystals formed at the seed points.

The diameter of the seed points is preferably smaller by a multiple than the diameter of the ice crystals.

According to a further embodiment, the seed points are punctiform.

Punctiform should be understood as meaning that the seed points only have a very small surface. The seed points may be for example circular, elliptical, oval, polygonal or star-shaped.

According to a further embodiment, the heat transfer tube comprises heat transfer ribs which are provided on the outer side of the tube portion and on which the coating is provided.

The heat transfer ribs preferably extend radially out from the tube portion. The coating is provided both on the tube portion and on the heat transfer ribs. The heat transfer ribs may be branched. This has the effect of increasing a surface of the heat transfer tube, whereby a good heat transfer from the fresh air to the cryogenic fluid is ensured.

According to a further embodiment, the tube portion is produced from an aluminum alloy.

The tube portion is preferably formed in one piece with the heat transfer ribs from the same material. For example, the tube portion may be an extruded profile. The use of an aluminum alloy for example means that very good heat transfer properties are ensured. Alternatively, the tube portion may be produced for example from a steel alloy, a fiber-composite material, a plastics material or any other material.

According to a further embodiment, the coating is a sol-gel coating.

In particular, the coating is a nano coating, or may be referred to as a nano coating.

According to a further embodiment, the coating is a single-component polysiloxane-urethane one-layer coating material.

For example, a polysiloxane-urethane resin may be used as a binder base, filled with nanoparticles. The coating may have for example a layer thickness of 3-10 μm after curing.

According to a further embodiment, the seed points are arranged evenly distributed in the hydrophobic component.

Alternatively, the seed points may be arranged unevenly distributed in the hydrophobic component. The seed points are preferably spaced apart from one another in such a way that the ice crystals formed do not contact one another before falling off from the heat transfer tube. As a result, the formation of a crust of ice is reliably prevented.

According to a further embodiment, the heat transfer tube also has a device for introducing shocks and/or vibrations into the heat transfer tube.

This makes it easier for the ice crystals to be removed. The device may for example be activated continuously or at regular intervals. The device may for example have a spring-biased hammer.

According to a further embodiment, the seed crystals are embedded in the hydrophobic component in such a way that a respective surface of the seed points is not covered by the hydrophobic component.

That is to say that the surface of the seed points is unwetted by the hydrophobic component.

Also proposed is an air-heated evaporator for heating and/or evaporating cryogenic fluids with at least one such heat transfer tube.

The air-heated evaporator may have a multiplicity of heat transfer tubes. The heat transfer tubes are preferably positioned vertically.

According to one embodiment, a number of heat transfer tubes are connected in series.

The heat transfer tubes are preferably connected to one another with the aid of tube bends. The heat transfer tubes may also be connected in parallel. The heat transfer tubes may be mounted on a supporting frame. The supporting frame may be fastened on a foundation, in particular a concrete slab.

Also proposed is a method for producing a heat transfer tube, in particular a finned tube, for an air-heated evaporator for heating and/or evaporating cryogenic fluids. The method comprises the following steps: providing a tube portion and coating the outer side of the tube portion with a coating, which has a hydrophilic component and a hydrophobic component, the hydrophilic component forming in the coating seed points, which are peripherally enclosed by the hydrophobic component, for condensing air moisture on the same and the seed points having a size of less than 100 nm.

The tube portion preferably comprises heat transfer ribs, which extend out radially therefrom. The tube portion may also not have any ribs, and therefore be smooth. The heat transfer ribs are likewise provided with the coating. The coating may be applied to the heat transfer tube or to the tube portion for example by means of a spraying method, a dipping method or a flooding method and the heat transfer ribs applied. The coating may be burned into the heat transfer tube at an elevated temperature.

Further possible implementations of the heat transfer tube, the air-heated evaporator and/or the method also comprise combinations of features or embodiments described above or hereinafter with respect to the exemplary embodiments that have not been mentioned explicitly. A person skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the heat transfer tube, the air-heated evaporator and/or the method.

Further advantageous refinements and aspects of the heat transfer tube, the air-heated evaporator and/or the method are the subject of the dependent claims and of the exemplary embodiments of the heat transfer tube, the air-heated evaporator and/or the method described hereinafter. The heat transfer tube, the air-heated evaporator and/or the method will be explained in more detail below by means of preferred embodiments with reference to the appended figures.

FIG. 1 shows a schematic side view of one embodiment of an air-heated evaporator;

FIG. 2 shows a further schematic side view of the air-heated evaporator according to FIG. 1;

FIG. 3 shows a schematic view of the air-heated evaporator according to FIG. 1;

FIG. 4 shows a schematic sectional view of an embodiment of a heat transfer tube for the air-heated evaporator according to FIG. 1;

FIG. 5 shows the view of a detail V according to FIG. 4;

FIG. 6 shows a schematic sectional view according to the sectional line VI-VI of FIG. 5, and

FIG. 7 shows a schematic block diagram of an embodiment of a method for producing a heat transfer tube according to FIG. 4.

In the figures, elements that are the same or have the same function have been given the same reference signs, unless stated otherwise.

FIG. 1 shows a schematic side view of an embodiment of an air-heated evaporator 1 for heating and/or evaporating cryogenic fluids. FIG. 2 shows a further schematic side view of the evaporator 1 and FIG. 3 shows a schematic plan view of the evaporator 1. In the following, reference is at the same time made to FIGS. 1 to 3.

Examples of cryogenic fluids or liquefied low-temperature gases are liquid oxygen, liquid nitrogen, liquid argon, liquid hydrogen, liquid carbon dioxide, liquid ethene or ethylene, liquid ethane, liquid helium or liquefied natural gas (LNG). The evaporator 1 may be used for example for heating and/or evaporating cryogenic fluids in the area of metal processing, medical engineering, electronics, water treatment, power generation, the food industry, environmental technology or similar areas. The evaporation 1 is designed to heat and/or evaporate cryogenic fluids with the aid of the heat of the surrounding air.

The evaporator 1 comprises a multiplicity of heat transfer tubes 2, only two of which are provided with a reference sign in each of FIGS. 1 to 3. There can be any number of the heat transfer tubes 2. For example, as shown in FIGS. 1 to 3, the evaporator 1 may have 30 heat transfer tubes 2. Alternatively, the evaporator 1 may also comprise only 24, 16, 12, 6 or 4 heat transfer tubes 2. A number of heat transfer tubes 2 may form a tube assembly 3 of the evaporator 1. As FIG. 3 shows, each tube assembly 3 may be assigned six heat transfer tubes 2 connected one behind the other. The heat transfer tubes 2 of a tube assembly 3 are fluidically connected to one another with the aid of tube bends 4. Furthermore, the tube assemblies 3 are also connected to one another with the aid of tube bends 4.

The evaporator 1 also has a first connection 5 and a second connection 6. For example, a cryogenic fluid may be introduced into the evaporator 1 through the connection 5, it flowing through all of the heat transfer tubes 2 of the evaporator 1 one after the other, to be removed again from the evaporator 1 in the heated or evaporated state out of the connection 6. During the operation of the evaporator 1, the heat transfer tubes 2 are passed over by fresh air L. The fresh air L gives off heat to the heat transfer tubes 2. The fresh air L is thereby cooled down and the heat transfer tubes 2 are heated.

The evaporator 1 also comprises a supporting framework 7, on which the tube assemblies 3 are arranged. The tube assemblies 3 may for example be screwed or welded to the supporting framework 7. The supporting framework 7 may be arranged on a foundation not shown in FIGS. 1 to 3, in particular a concrete foundation. Without the supporting framework 7, the evaporator 1 may have a height h₁ of for example 3 to 6 m. The evaporator 1 may also have a width b₁ of for example 30 cm to 2 m. The evaporator 1 may also have a depth t₁ of example 50 cm to 1.5 m.

FIG. 4 shows a schematic sectional view of one embodiment of a heat transfer tube 2. The heat transfer tube 2 has a tube portion 8, through the interior space 9 of which a cryogenic fluid is passed. To increase the surface area, a multiplicity of first heat transfer ribs 10 protrude into the interior space 9. As a result, the heat transfer from the tube portion 8 to the cryogenic fluid is improved. Provided on the outer side of the tube portion 8 are second heat transfer ribs 11, extending out radially therefrom. The tube portion 8, the first heat transfer ribs 10 and the second heat transfer ribs 11 are preferably formed in one piece from the same material. For example, the heat transfer tube 2 is an extruded profile. The heat transfer tube 2 is preferably produced from an aluminum material.

The second heat transfer ribs 11, provided on the outer side of the tube portion 8, may comprise branches 12, 13 or end portions 14 provided at the ends of the second heat transfer ribs 11. With the aid of the branches 12, 13 and/or the end portions 14, an increase in the surface area of the heat transfer tube 2 can be achieved. The heat transfer ribs 10, 11 may be arranged evenly distributed over a circumference of the tube portion 8. Since the second heat transfer ribs 11 extend out from the tube portion 8 in the form of fins, the heat transfer tube 2 is also referred to as a finned tube. The tube portion 8 may also not have any ribs, and therefore be smooth. The heat transfer tube 2 also has a coating not shown in FIG. 4 that is applied to the outer side.

FIG. 5 shows a greatly enlarged schematic plan view of the heat transfer tube 2 according to the view of a detail V of FIG. 4. FIG. 6 shows a schematic partial sectional view of the heat transfer tube 2 according to the sectional line VI-VI of FIG. 5. In the following, reference is at the same time made to FIGS. 5 and 6.

Provided on the outer side of the tube portion 8 and the second heat transfer ribs 11 is a coating 15. The coating 15 is preferably what is known as a sol-gel coating. A sol-gel coating should be understood as meaning an inorganic or hybrid-polymer film system produced by means of a sol-gel process. A hybrid polymer should be understood as meaning a polymeric material that combines structural units of various material classes at a molecular level. A sol-gel process is a process for producing nonmetallic inorganic or hybrid-polymer materials from colloidal dispersions, known as sols. The starting materials are also referred to as precursors. Extremely fine particles are produced from these precursors in solution in first basic reactions. A special further processing of the sols allows powders, fibers, layers or aerogels to be created. Because of the small size of the initially created sol particles in the nanometer range, the sol-gel process can be understood as part of chemical nanotechnology.

In particular, the coating 15 may be a single-component polysiloxane-urethane one-layer coating material that is filled with nano particles. A polysiloxane-urethane resin may be used here as a binder. The coating 15 may be applied to the heat transfer tube 2 with the aid of a spraying, dipping or flooding method. The coating 15 may be burned in at elevated temperature. As shown in FIG. 6, the coating 15 may have a thickness d₁₅ of 3 to 10 μm after curing. The coating 15 has a hydrophilic component 16 and a hydrophobic component 17. Hydrophilicity means water-loving, which indicates that a substance interacts strongly with water or other polar substances. The opposite of hydrophilicity is hydrophobicity. Hydrophobicity literally means water-avoiding. In chemistry, hydrophobic is used to characterize substances that do not mix with water and allow water to form droplets on surfaces. If a surface is strongly water-attracting, one also speaks of superhydrophilicity. Preferably, the portion 16 is superhydrophilic. Only the hydrophilic portion 16 can be wetted with water. In particular, the hydrophilic portion 16 can be completely wetted with water. The hydrophobic portion 17 cannot be wetted with water.

The hydrophobic portion 16 forms in the coating 15 seed points 18 that are peripherally enclosed by the hydrophobic portion 17. The seed points 18 are preferably punctiform. The seed points 18 may also be referred to as particles. The seed points 18 are nanoparticles. The seed points 18 have a particle size of less than 100 nm. In the area of particle technology and particle measuring technology or dispersity analysis, the equivalent diameter of a particle is chosen as a feature. The general frequency distribution of statistics therefore gives the particle size distribution. This is often also referred to as grain size distribution. The equivalent diameter is a measure of the size of an irregularly formed particle, such as for example a grain of sand. It is calculated from the comparison of a property of the irregular particle with a property of a regularly formed particle.

The seed points 18 may have any geometry. As shown in FIGS. 5 and 6, the seed points 18 may for example have a circular cross section. Alternatively, the seed points 18 may also be oval, elliptical, polygonal, star-shaped or the like. The seed points 18 may be arranged evenly or unevenly distributed in the hydrophobic component 17. For example, the seed points 18 have a diameter d₁₈. The diameter d₁₈ may be the equivalent diameter of the seed points 18. The diameter d₁₈ is preferably less than 100 nm, more preferably less than 90 nm, more preferably less than 80 nm. The seed points 18 are embedded in the hydrophobic component 17 in such a way that a respective surface 19 of the seed points 18 is not covered by the hydrophobic component 17.

As shown in FIG. 6, the seed points 18 may be embedded in the hydrophobic component 17 in such a way that they do not contact the tube portion 8. Alternatively, the seed points 18 may also contact the tube portion 8. The seed points 18 form crystallization points for the formation of ice crystals 20 on the heat transfer tube 2. The air moisture contained in the fresh air L condenses on the heat transfer tube 2 and freezes on it. With the aid of the seed points 18, a controlled condensation and crystallization of the air moisture contained in the fresh air L is achieved.

The surface 19 of the seed points 18 is in this case so small that ice crystals 20 forming at the seed points 18 are spherical. That is to say that the ice crystals 20 only contact the coating 15 at the seed points 18, and consequently with a very small surface. The ice crystals 20 keep growing until they fall off from the heat transfer tube 2 under their own weight or are removed by the fresh air L flowing over the heat transfer tube 2. The formation of an insulating layer, such as for example a continuous crust of ice, on the heat transfer tube 2 is prevented by the early removal of the ice crystals 20. The diameter d₁₈ of the seed points 18 is in this case smaller than a diameter d₂₀ of the ice crystals 20. Since the hydrophobic component 17 cannot be wetted with water, no ice crystals 20 form on it either.

To reliably ensure that the ice crystals 20 fall off from the heat transfer tube 2, the heat transfer tube 2 may have a device 21 that is only indicated in a very simplified manner in FIG. 1 for introducing shocks, vibrations and/or oscillations into the heat transfer tube 2. The device 21 may for example comprise a spring-biased hammer. The device 21 may for example subject the heat transfer tube 2 to shocks, vibrations and oscillations continuously or at regular intervals, so that the ice crystals 20 come away from the heat transfer tube 2

The heat transfer tube 2 has the following advantages over known heat transfer tubes. Using the seed points 18 to bring about the specifically intended condensation of the air moisture contained in the fresh air L achieves the formation of spherical ice crystals 20, which keep growing until their own weight is so great that they either fall off from the heat transfer tube of their own accord, are carried away by the air flow of the fresh air L or can be easily detached from the heat transfer tube 2 by introducing shocks, vibrations and/or oscillations with the aid of the device 21. The specifically intended formation of the ice crystals 20 has the effect of preventing the formation of an insulating layer, such as for example a crust of ice, on the heat transfer tube 2. As a result, a good heat transfer from the fresh air L to the cryogenic fluid is always ensured.

FIG. 7 shows a schematic block diagram of one embodiment of a method for producing such a transfer tube 2. In a step S1, the tube portion 8 with the heat transfer ribs 10, 11 is provided. In a step S2, the tube portion 8 provided on the outer side with the coating 15, which has the hydrophilic component 16 and the hydrophobic component 17, the hydrophilic component 16 forming in the coating 15 seed points 18, which are peripherally enclosed by the hydrophobic component 17, for condensing air moisture from the fresh air L on the same. In this case, the seed points 18 have a diameter d₁₈ of less than 100 nm. Nanoparticles are used here as seed points 18.

Although the present invention has been described using exemplary embodiments, it can be modified in various ways.

REFERENCE SIGNS USED

-   1 Evaporator -   2 Heat transfer tube -   3 Tube assembly -   4 Tube bend -   5 Connection -   6 Connection -   7 Supporting framework -   8 Tube portion -   9 Interior space -   10 Heat transfer rib -   11 Heat transfer rib -   12 Branching -   13 Branching -   14 End portion -   15 Coating -   16 Hydrophilic component -   17 Hydrophobic component -   18 Seed point -   19 Surface -   20 Ice crystal -   21 Device -   b₁ Width -   d₁₅ Thickness -   d₁₈ Diameter -   d₂₀ Diameter -   h₁ Height -   L Fresh air -   S1 Step -   S2 Step -   t₁ Depth 

1. A heat transfer tube (2), in particular a finned tube, for an air-heated evaporator (1) for heating and/or evaporating cryogenic fluids, with a tube portion (8) and a coating (15), which is provided on the outer side of the tube portion (8) and has a hydrophilic component (16) and a hydrophobic component (17), the hydrophilic component (16) forming in the coating (15) seed points (18), which are peripherally enclosed by the hydrophobic component (16), for condensing air moisture on the same and the seed points (18) having a size of less than 100 nm.
 2. The heat transfer tube as claimed in claim 1, the seed points (18) being so small that ice crystals (20) formed at the seed points (18) are spherical.
 3. The heat transfer tube as claimed in claim 2, the seed points (18) being so small that the ice crystals (20) fall off from the heat transfer tube (2) under their own weight.
 4. The heat transfer tube as claimed in claim 2, the diameter (d₁₈) of the seed points (18) being smaller than a diameter (d₂₀) of the ice crystals (20) formed at the seed points (18).
 5. The heat transfer tube as claimed in claim 1, the seed points (18) being punctiform.
 6. The heat transfer tube as claimed in claim 1, also having heat transfer ribs (11), which are provided on the outer side of the tube portion (8) and on which the coating (15) is provided.
 7. The heat transfer tube as claimed in claim 1, the tube portion (8) being produced from an aluminum alloy.
 8. The heat transfer tube as claimed in claim 1, the coating (15) being a sol-gel coating.
 9. The heat transfer tube as claimed in claim 8, the coating (15) being a single-component polysiloxane-urethane one-layer coating material.
 10. The heat transfer tube as claimed in claim 1, the seed points (18) being arranged evenly distributed in the hydrophobic component (17).
 11. The heat transfer tube as claimed in claim 1, also having a device (21) for introducing shocks and/or vibrations into the heat transfer tube (2).
 12. The heat transfer tube as claimed in claim 1, the seed points (18) being embedded in the hydrophobic component (17) in such a way that a respective surface (19) of the seed points (18) is not covered by the hydrophobic component (17).
 13. An air-heated evaporator (1) for heating and/or evaporating cryogenic fluids, with at least one heat transfer tube (2) as claimed in claim
 1. 14. The air-heated evaporator as claimed in claim 12, a number of heat transfer tubes (2) being connected in series.
 15. A method for producing a heat transfer tube (2), in particular a finned tube, for an air-heated evaporator (1) for heating and/or evaporating cryogenic fluids, with the following steps: providing (S1) a tube portion (8); and coating (S2) the outer side of the tube portion (8) with a coating (15), which has a hydrophilic component (16) and a hydrophobic component (17), the hydrophilic component (16) forming in the coating (15) seed points (18), which are peripherally enclosed by the hydrophobic component (17), for condensing air moisture on the same, and the seed points (18) having a size of less than 100 nm. 